For many years, the familiar “horse head”, walking beam-type mechanism has been used for pumping fluids such as water and/or oil from subterranean formations. An example of such a walking beam apparatus 50, connected to a polished rod 52 extending from a well head 54 of a well 56, is illustrated as prior art in the attached FIG. I.
Conventional walking beam apparatuses have a number of disadvantages, not the least of which is their large size. In addition, performance of the walking beam pump apparatus is largely a function of the design and connection of a number of mechanical parts, which include massive counter-weights and complex drive mechanisms which are difficult to control for obtaining maximum pumping efficiency or to compensate for changes in condition of the well over time.
As shown in FIG. 1, because of their large size and weight, walking beam-type pumping mechanisms must typically be mounted on a heavy concrete foundation 58, which may be poured in place or pre-cast, located adjacent the well head 54. Construction of a walking beam pumping mechanism, together with its foundation, typically involves the efforts of several construction workers, over a period which may be a week or more, to prepare the site, lay the foundation 58, and allow time for the foundation 58 to cure, in addition to the time required for assembling the various components of the walking beam mechanism 50 onto the foundation 58 and operatively connecting the mechanism to the polished rod 52. In general, because of the costs of transporting the apparatus and the concrete or pre-cast foundation to what may be a remote site and the complexity of the site preparation and assembly process, walking beam-type pumping mechanisms are generally only utilized in long-term pumping installations.
The large size and massive weight of the walking beam pumping mechanism and its foundation are also problematic when the well 56 is decommissioned. Economic and contractual obligations may require complete removal of the walking beam mechanism and its foundation. It is desirable, therefore, to provide an improved apparatus and method for operating the well 56, which eliminates, or at least greatly reduces, the significant expenditures in time, manpower, and money required to install and remove a pumping apparatus used for extracting fluid from the well 56.
Another disadvantage of walking beam-type pumping apparatuses is that they cannot typically operate at pumping speeds much below 5 strokes per minute. As a result, it has been necessary in the past, to only pump intermittently or to decommission wells which could not sustain pumping at rates of at least 5 strokes per minute, even though such wells would be capable of continued operation at lower pumping speeds. Intermittent pumping creates problems caused by varying levels of fluid in the well casing and tubing and collection of contaminants into the pump during “off” periods. As mentioned above, decommissioning a well equipped for pumping with a walking beam-type mechanism is an arduous and costly task. Further, government regulations frequently require the costly process of sealing the well 56 with cement or other sealing means when a well is decommissioned. It would be desirable, therefore, to provide an improved apparatus and method, for pumping fluid from the well 56, which could operate at considerably slower pumping rates than a walking beam-type mechanism, in a form that could be connected to the polished rod 52 in place of a walking beam mechanism 50, at an existing well 56, to thereby extend the useful life of the well 56 by operation at a pumping speed lower than could otherwise be accomplished by the walking beam-type apparatus. If such an improved pumping apparatus and method were available in a form that could be quickly and simply installed on an existing well 56, the necessity for, and cost related to, decommissioning the well, and in particular the cost related to sealing the well and removal of the walking beam mechanism and its foundation could be deferred, perhaps indefinitely, while the well 56 is operated at a low pumping rate.
Because of their large size and complexity, walking beam-type pumping mechanisms typically need to be shut-down and repaired on site. Although there have been attempts in the past to develop portable walking beam apparatuses, such as those described in U.S. Pat. No. 4,788,873, to Laney, such portable walking beam pumping apparatuses have not gained widespread acceptance in the art. It would be desirable, therefore, to have an improved pumping apparatus and method, in which the pumping apparatus could be readily transported to a well, and quickly installed in place of an existing walking beam apparatus, or another one of the improved pumping apparatuses previously attached to the well, to thereby substantially reduce downtime of the well during the process of performing maintenance and/or repairs of the pumping apparatus. It would also be desirable for such an improved pumping apparatus and method to allow for convenient installation and/or removal of the improved pumping apparatus, substantially in a completely assembled form, which could be initially assembled, or repaired, offline, at a location remote from the well, while the well was continuing to operate with another of the improved pumping apparatuses.
Another problem inherent in the use of walking beam-type pumping apparatuses is that the apparatus must typically extend a substantial distance above ground level in order to achieve a desired pumping stroke length on the order of 3 to 6 feet. At such substantial heights it may be difficult, if not impossible, to operate irrigation equipment, for example, in close proximity to the walking beam pumping apparatus, where such irrigation equipment must pass over the top of the walking beam apparatus. U.S. Pat. No. 6,015,271, to Boyer et al. discloses a stowable walking beam pumping unit having a foldable support structure to allow storage of the pumping unit in a low profile position. A stowable walking beam pumping unit, as disclosed by Boyer, has not been shown to be commercially viable, however. It is desirable, therefore, for an improved pumping apparatus and method to be operable in a form having a low enough profile that other equipment, such as irrigation pipes mounted on rolling supports can safely pass above the pumping apparatus.
U.S. Pat. No. 4,114,375, to Saruwatari discloses replacing the conventional walking beam pumping apparatus with a pump jack device including a double acting piston and cylinder motor, with the piston rod of the motor being adapted to be connected to the polished rod projecting upwardly from a well head. A variable displacement hydraulic pump, driven by a motor or engine, is included in a closed hydraulic loop wherein conduits are connected to a pair of output ports of the pump. A pump control means controls the direction and volume of flow in the loop so as to establish the stroke of the piston rod. A compressible fluid counter-balance is provided for accumulation of energy during a down stroke of the piston rod so that the energy may be returned to the piston during the upstroke. The counter-balance cylinder may be mounted coaxially above the motor and an additional closed chamber may be provided in fluid communication with a charged chamber of the counter-balance.
To date, the apparatus of Saruwatari has not achieved commercial success.
Regardless of the type of pumping apparatus utilized, controlling and optimizing the performance of a sucker-rod pumping apparatus involves inherent difficulties. One factor which must be taken into account is the stretching of the rod string, which occurs during the upward portion of each pump stroke, and the corresponding contraction of the rod string which occurs during the downward portion of each pump stroke. The rod string, which may be 1000 feet or more long, acts much like an extension spring, which is stretched during the portion of the pump stroke in which the rod string is drawing the fluid upward within the well, and which then contracts back to an essentially un-stretched state as the rod string moves downward during a return portion of the pump stroke. As a result of the rod stretch, an above-ground upward stroke of 32 inches, for a well approximately 1300 feet deep, may only result in a down-hole stroke in the range of 24 to 26 inches, for example. The difference between the magnitude and direction of movement of the polished rod at the top of the well and the corresponding reaction of the rod string and down-hole stroke of the pump involves other complicating factors, including inherent damping within the rod string, fluid damping which occurs during the pump stroke and longitudinal vibrations and natural frequencies of the rod string.
An additional difficulty occurs where the fluid being pumped upward from the well contains a significant amount of entrained gas. In such circumstances, a suction effect during the upward stroke of the rod string causes the entrained gas to bubble out of the fluid and form a foamy segment at the top of the column of fluid being pulled upward toward the surface through action of the down-hole components of the sucker-rod pump. Specifically, a typical down-hole pump portion of a sucker-rod pump, apparatus is located at the bottom of a length of tubing terminating in a fluid outlet above the surface of the ground and includes a standing valve, located at the lower end of the down-hole pump, and a traveling valve, which is attached to the bottom end of the rod string and is movable by the rod string within the down-hole pump above the standing valve. The standing valve performs a check-valve function which allows fluid to flow into the lower end of the down-hole pump when the pressure within the down-hole pump is lower than the pressure in the well casing outside of the down-hole pump. When pressure within the down-hole pump is equal to, or greater than, the pressure outside of the down-hole pump, the check-valve function of the standing valve closes to preclude movement of fluid out of the down-hole pump through the standing valve. The traveling valve also includes a check-valve function, which works substantially oppositely to the check-valve function of the standing valve. When the pressure within the down-hole pump below the traveling valve is lower than the pressure within the tubing above the traveling valve, the traveling valve is closed. Conversely, when the pressure within the down-hole pump below the traveling valve is greater than the pressure within the tubing above the traveling valve, the traveling valve opens and allows fluid movement through the traveling valve, so that the traveling valve can descend through the fluid in the down-hole pump.
By virtue of this arrangement, as the rod string pulls the traveling valve upward, during the upward portion of the pump stroke, the traveling valve is closed, and the upward motion of the traveling valve within the tubing generates a suction in the down-hole pump below the traveling valve which causes the standing valve to open and allow fluid to be drawn upward into the portion of the down-hole pump between the standing and traveling valves. Where the sucker-rod pump is pumping a fluid with no entrained gas, as soon as the rod string begins the downward portion of its stroke, the standing valve closes and the stationary valve opens, to thereby trap fluid within the down-hole pump above the standing valve, and allow the traveling valve to move downward through the trapped fluid within the down-hole pump, toward the standing valve, to the bottom of the pump stroke, where the rod string reverses direction and begins to pull the traveling valve upward at the start of the next pump stroke.
For the above-mentioned exemplary well, pumping water for dewatering coal bed methane and having a depth of approximately 1300 feet, the fluid load being moved upward by each stroke of the pump once the entire length of tubing has been filled, for example, would be 5400 pounds, and the weight of the rod string would be approximately 1800 pounds. As a result, during each stroke of the pump, the load on the rod string varies approximately by the 5400 pound fluid load, which causes a significant change in the length of the rod string, as the rod string stretches and contracts during each pump stroke. Fluid damping effects which occur as a result of the movement of the traveling valve upward and downward through fluid within the tubing and viscous effects related to the flow of the fluid upward within the tubing also affect the dynamic performance of the rod string.
Other complications also occur in wells having a fluid in the form of a liquid having entrained gas. In these wells, the traveling valve does not open immediately as it begins the downward portion of its movement within the down-hole pump, due to the presence of the foamy portion of the fluid column existing between the traveling valve and the liquid portion of the fluid column. The traveling valve must travel downward in the down-hole pump some distance while compressing the gas which has foamed out of the fluid before the suction effect dissipates to the point where the pressure difference across the traveling valve is such that the traveling valve can open.
As will be readily understood by those having skill in the art, accurately predicting the down-hole performance of the sucker-rod pump for a given input at the polished rod above the surface of the ground is a challenging design problem, with the specific difficulties discussed briefly above being far from totally inclusive.
The problems of effectively and efficiently operating a sucker-rod pump apparatus are addressed in significantly greater detail in a commonly assigned U.S. Pat. No. 7,168,924 B2, to Beck et al., titled “Rod Pump Control System Including Parameter Estimator.” The Beck et al. patent also discloses a rod pump control system, which includes a parameter estimator that determines, from motor data, parameters relating to operation of the rod pump and/or generating a down-hole dynamometer card, without the need for external instrumentation such as down-hole sensors, rod load sensors, flow sensors, acoustic fluid level sensors, etc. In some embodiments disclosed by Beck et al., having a pumping apparatus driven by an electric motor, instantaneous current and voltage, together with pump parameters estimated through the use of a computer model of the sucker-rod pump, are used in determining rod position and load. The rod position and load are used to control the operation of the rod pump to optimize operation of the pump. Beck et al. also discloses a pump-stroke amplifier that is capable of increasing pump stroke without changing the overall pumping speed, or in the alternative, maintaining the well output with decreased overall pumping speed.
The commonly assigned Beck et al. patent, also provides a detailed description of the considerable additional complexity involved in operating a sucker-rod pump with a walking beam pumping apparatus, or with prior belt driven pumping units, and further provides a method and apparatus for efficiently and effectively controlling a sucker-rod pumping apparatus having a rod string driven by a walking beam pumping apparatus, or other types of previously-known pumping apparatuses.
With regard to the present invention, the detailed descriptions within Beck et al., of the manner in which the inherent difficulties of operating a sucker-rod pump apparatus are compounded by a complex pumping apparatus such as the typical walking-beam-type apparatus serve as ample evidence of the desirability of providing a new and improved pumping apparatus for use with a sucker-rod pump, which is not subject to the multitude of complexities involved in controlling prior pumping apparatuses such as the typical walking-beam-type pumping apparatus.
Even though the performance of walking-beam pump and other types of prior pumping apparatuses can be substantially improved through practicing the teachings of Beck et al., it is, therefore, still highly desirable to provide an improved apparatus and method for use in pumping fluids such as water and/or hydrocarbons from subterranean formations and reservoirs in a form overcoming problems such as, and in addition to, those discussed above. It is further desirable that such improvements be provided in a form which is considerably smaller in physical size than conventional walking beam apparatuses and also in a form which is less complex and more readily controllable and/or adjustable than prior conventional walking beam-type apparatuses. It is further desirable that such an improved apparatus and method provide advancements over the pump jack device of Saruwatari, in a form that is commercially viable.